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In 1750, the electrical nature of lightning was the subject of public discussion in France, with a dissertation of Denis Barbaret receiving a prize in Bordeaux; Barbaret proposed a cause in line with the triboelectric effect. The physician Jacques de Romas also wrote a memoir that year with similar ideas. Franklin had listed a dozen analogies between lightning and electricity in his notebooks at the end of 1749.[1] Speculations of Jean-Antoine Nollet had led the issue being posed as a prize question at Bordeaux in 1749. De Romas later defended his own electrical kite proposal as independent of Franklin's.[2]

Such an experiment was carried out in May 1752 at Marly-la-Ville in northern France [3] by Thomas-François Dalibard. An attempt to replicate the experiment killed Georg Wilhelm Richmann in Saint Petersburg in August 1753, thought to be the victim of ball lightning.[4] Franklin himself is said to have conducted the experiment in June 1752, supposedly on the top of the spire on Christ Church in Philadelphia. However, doubts have been expressed about whether the experiment was actually performed (see below).

The BEPengraved the vignette Franklin and Electricity (c. 1860) which was used on the $10 National Bank Note from the 1860s to 1890s.

Franklin realized the dangers of using conductive rods and instead used a kite. The increased height allowed him to stay on the ground and the kite was less likely to electrocute him. According to the legend, Franklin kept the string of the kite dry at his end to insulate him while the rest of the string was allowed to get wet in the rain to provide conductivity. A house key was attached to the string and connected to a Leyden jar, which Franklin assumed would accumulate electricity from the lightning. The kite wasn't struck by visible lightning (had it done so, Franklin would almost certainly have been killed) but Franklin did notice that the strings of the kite were repelling each other and deduced that the Leyden jar was being charged. Franklin reportedly received a mild shock by moving his hand near the key afterwards, because as he had estimated, lightning had negatively charged the key and the Leyden jar, proving the electric nature of lightning.[5]

Fearing that the test would fail, or that he would be ridiculed, Franklin only took his son to witness the experiment, and then published the accounts of the test in third person.[6]

De Romas pursued his priority claim to the kite experiment. He had it recognised by the Bordeaux Academy, and then the Académie française in Paris.[3]

The standard account of Franklin's experiment was disputed following an investigation and experiments based on contemporaneous records by science historian Tom Tucker, the results of which were published in 2003.[7] According to Tucker, Franklin never performed the experiment, and the kite as described is incapable of performing its alleged role.[8]

Further doubt about the standard account has been cast by an investigation by the television series MythBusters. The team found evidence that Franklin would have received a fatal current through his heart had the event actually occurred. Nevertheless, they confirmed that certain aspects of the experiment were feasible - specifically, the ability of a kite with sufficiently damp string to receive and send to the ground the electrical energy delivered by a lightning strike.[9]

Because of the danger inherent in the kite experiment, it has become illegal in certain locales.[10]